1. Field of the Invention
This invention relates to a powder composition for producing a high-density and high-strength sintered ceramic, and a process for producing such a powder composition. More particularly, the invention relates to a powder composition with which it is possible to produce green moldings with a high density and few molding defects under a relatively low molding pressure, and a process for producing such a composition.
2. Description of the Prior Art
In the prior art for producing sintered ceramic, a powdered raw material is first molded into a green molding of a desired shape and then this green molding is sintered. Various methods are known for molding the powdered raw material into a green molding of a desired shape, such as slip cast molding, extrusion molding, potter's wheel molding, rock molding and dry pressure molding. These molding methods may be roughly divided into a wet process and a dry one depending on the moisture content of the molding material during molding. In the wet molding methods, sufficient drying of the green molding is necessary, so that in these methods it is not only required to give consideration to the possible shrinkage due to drying but also there tends to occur warping or cracking of the molding during drying. On the other hand, the dry molding methods are advantageous in providing an excellent dimensional precision, but in case of using a fine powder, it is found difficult to uniformly fill and press the material in a mold because of poor fluidity of the powder. These methods, therefore, tend to cause scattering of the bulk density of the green molding or pressure distribution during molding and to invite defects originating in or associated with bridging in the green molding. The defects incurred during molding remain in the sinter to adversely affect its properties, especially mechanical strength. Therefore, in using a finely powdered material for forming a green molding, it has been usual to granulate the material in advance for bettering its fluidity.
From the viewpoint of handling ease such as fluidity, it is desirable that the granules are high in bulk density and hard, but in order to obtain a high-density and high-strength sinter by pressure molding granulated material, it is important that individual granules are completely crushed in the course of the pressure molding so that an integral green molding is formed. To this end, usually granules which are relatively low in bulk density and soft are required because those of a high bulk density and a high hardness are hard to crush so that an extremely high molding pressure is required in molding such granules. Further, voids tend to remain in the green molding, making it difficult to obtain a high-strength sinter.
However, granules of too low a bulk density tend to break into powders again when handled, and because of an excessively high compression ratio at the time of molding, lamination tends to occur due to the air trapped in the mold. Also, a long time is required for the molding. Thus, an efficient production of green moldings is difficult with these granules.
As a means for improving these points, it is a common practice to use a lubricant for bettering the transmission of molding pressure and thereby improving the moldability.
As the lubricant, the following materials are known usable: carbowax, diglycol stearate, stearic acid, magnesium stearate, zinc stearate, barium stearate, aluminum stearate, a mixture of mineral oil and fat, paraffin emulsion, wax emulsion, glycerol or polyethylene wax.
For example, Japanese Patent Laid-Open No. 78609/1975, claiming a priority based on its original U.S. patent application Ser. No. 409,073 of Oct. 24, 1973, discloses a method in which aluminum stearate is added as a lubricant in pressure molding silicon carbide of a submicron particle size to form a green molding. According to this method, however, the green molding formed under a molding pressure of 5,000 p.s.i. (approximately 351.5 kg/cm.sup.2) by a die press has a density corresponding to 55% of the theoretical value, and the green molding formed by a hydrostatic press under a molding pressure of 30,000 p.s.i. (approximately 2,109 kg/cm.sup.2) has a density corresponding to 59% of the theoretical value, so that with this method an extremely high molding pressure is required for obtaining a high-density green molding.
As discussed above, a ceramic sintering powder for use in a dry molding method is required to have an excellent moldability for obtaining a high-density sinter and, for this purpose, such a powder is usually incorporated with a lubricant and granulated. In case an extremely fine powder is used as the ceramic sintering powder and granulated, the obtained granules prove to be very hard to break because of strong interparticle cohesion, and use of a hitherto known lubricant can not provide a significant improvement in the moldability. Thus, it has been difficult to mold an extremely fine ceramic sintering powder by a conventional dry molding technique to obtain a high-density and high-strength sinter.